Various pharmaceutically acceptable salts of fexofenadine have been disclosed, for example EP 1178041A, page 5, paragraph 0010. EP 1178041A further describes the preparation of anhydrous fexofenadine hydrochloride (Forms I and III) and hydrated forms (Forms II and Form IV). The process described in EP 1178041A involves making the anhydrous Forms I and III from the hydrated Forms II and IV, for example by azeotropic distillation thus heating in acidic conditions for extended periods. Extended heating in this way can cause following impurities I and II to increase beyond acceptable levels

EP 1178041A also describes a water-limiting crystallization that involves crystallization of the hydrate, but this involves using large volumes of solvent so as to limit the water content of the solution to acceptable levels.
WO 01/94313 describes Form A of fexofenadine hydrochloride. The process for its preparation involves the use of water, which means Form A is prepared as a hydrated form, which is not suitable for pharmaceutical formulation.
U.S. Pat. No. 4,254,129 is the basic fexofenadine patent and describes fexofenadine HCl being isolated from aqueous hydrochloric acid, which will lead to the formation of a hydrated form, which as indicated above is not suitable for pharmaceutical formulation.
WO 93/21156 also relates to fexofenadine hydrochloride and Example 1 describes dissolving fexofenadine base in methylene dichloride and acidifying with HCl gas to pH 3. The reaction mass is then concentrated to residue, ether is added and the mixture is stirred to obtain solid, which is filtered to give fexofenadine hydrochloride. This process leads to the formation of amorphous fexofenadine hydrochloride, which has a tendency to pick up moisture and form lumps on storage, and as such is difficult to handle during formulation of tablets and capsules. Furthermore, the use of ether as described in the preparation thereof is not advisable on a commercial scale and hence the process described therein is unsuitable for industrial application.
Apart from the hydrochloride salt, in anhydrous and hydrated forms as discussed above, however, no other salt of fexofenadine has been exemplified in the prior art. Conversion of hydrated salts to their anhydrous forms by water minimizing crystallization is also claimed in EP 766668B, but the disclosure is limited to the inter-conversion of corresponding salts. The conversion of one salt, hydrated or otherwise, to a different anhydrous salt form is not reported or suggested.